Active joystick with optical positions sensor

ABSTRACT

A joystick composed of a stator formed by an outer cage forming an inner cubic compartment containing an inner cube oriented with its wall spaced from and substantially parallel corresponding wall of the compartment. Opposed magnets are position in cooperating relationship on opposed walls of the compartment and cube and define a gap therebetween. A floater formed by a plurality of flat actuating coils, one positioned in each gap and each thinner than the width of the gap in which it is received. Preferably the ratio of coil thickness to gap width is at least 1:3. Preferably an optical position sensor is used to monitor the relative position of the flotor and stator and is composed of at least one linear light position sensor mounted on one of the stator and flotor and a plurality of planar light beams arranged at an angle to each other on the other of the stator and flotor and directed to the linear light position sensor(s) so that the light beams traverse the linear light position sensor(s).

This application claims benefit of Provisional application No.60/065,787, filed Nov. 14, 1997.

FIELD OF THE INVENTION

The invention relates to a motion device formed by a flat actuating coil(e.g. a Lorentz voice coil) positioned between magnets having improvedrattle space, a joystick with improved geometry incorporating the motiondevice and to an optical position sensing system

BACKGROUND TO THE PRESENT INVENTION

A number of active joysticks or haptic interfaces (interfaces capable ofgenerating kinesthetic and tactile feedback to the user) have beenproposed for virtual environments and teleoperation systems.

Attention is directed to Stocco, L., Salcudean, S. E., “A coarse-fineapproach to force-reflecting hand-controller design,” in Proc. 1996 IEEEIntl. Conf. Rob. Aut. Minneapolis, Minn., pp.˜404-410, Apr. 22-28 1996.for a detailed survey, and to Hayward, V., Astley, O. R., “Performancemeasures for haptic interfaces,” in Proc. ISRR, p.˜(12 pages), 1995. forperformance measures.

The need for high acceleration in haptic computer-user interfaces hasbeen demonstrated in many studies and seems to have been accepted bydesigners. Although most reported designs have translational workspacesthat exceed a cube with 10 cm sides, it has not been shown that aworkspace of this magnitude is really needed. Indeed, for desk-topcomputing, input devices such as mice, trackballs or joysticks arecommonplace. These devices have relatively small motion ranges to avoidtiring the operator. Furthermore, designing high acceleration devicesover large workspaces is a non-trivial task requiring expensiveactuators, transmissions and joints.

As an alternative, the use of a small workspace haptic device in ratemode or combined position/rate mode has been proposed and demonstratedsee Salcudean, S. E., Wong, N. M., Hollis, R. L., “Design and Control ofa Force-Reflecting Teleoperation System with Magnetically LevitatedMaster and Wrist,” IEEE Trans. Rob. Aut., vol.˜11, pp.˜844-858, December1995.

Magnetically levitated (maglev) Lorentz devices such as those describedin Hollis, R. L., Salcudean, S. E., Allan, P. A., “A sixdegree-of-freedom magnetically levitated variable compliance fine motionwrist: Design, modeling and control,” IEEE Trans. Rob. Aut., vol.˜7,pp.˜320-332, June 1991 and U.S. Pat. No. 5,146,566, issued September,1992 to Hollis, R. L. and Salcudean S. E are suitable small-motionhaptic interfaces because of their low mass, lack of friction andbacklash, and high acceleration ability. Devices have been built at IBM(see Hollis, R. L., Salcudean, S. E., Allan, P. A., “A sixdegree-of-freedom magnetically levitated variable compliance fine motionwrist: Design, modeling and control,” IEEE Trans. Rob. Aut., vol.˜7,pp.˜320-332, June 1991); at University of British Columbia (seeSalcudean, S. E., Wong, N. M., Hollis, R. L., referred to above), and atCarnegie-Mellon University (see Berkelman, P. J., Butler, Z. H., Hollis,R. L., “Design of a hemispherical magnetic levitation haptic interfacedevice,” in Proc. 1996 ASME IMECE, vol.˜DSC-58, Nov. 17-22 1996).

In such devices magnetic forces are used to actively levitate a rigidmass or flotor to which the handle manipulated by the operator isattached. These devices share the following three subsystems:

(i) an actuation system consisting of at least six flat voice-coil orLorentz actuators,

(ii) an optical position sensing system consisting of infrared linearlight rays projecting from light-emitting diodes or lasers ontotwo-dimensional lateral effect photodetectors or position sensingdiodes,

(iii) a control system that commands forces and torques to the actuationsystem based on the desired and sensed position, the desired force andthe desired relationship between force and position (mechanicalimpedance).

A number of applications of maglev devices are described in the surveypaper Hollis, R. L., Salcudean, S. E., “Lorentz levitation technology: anew approach to fine motion robotics, teleoperation, haptic interfaces,and vibration isolation,” in Proc. 5th Intl. Symp. on Robotics Research,(Hidden Valley, Pa.), p.˜(18 pages), Oct. 1-4 1993.

U.S. Pat. No. 5,790,108 issued to Salcudean et al. on Aug. 4, 1998describes a specific application of the Lorentz voice coils in a handcontroller.

BRIEF DESCRIPTION OF THE PATENT INVENTION

It is object of the present invention to provide voice-coil actuatorswith significantly larger “rattle space” or significantly higher forcesfor a given “rattle space”.

Broadly the present invention relates to a basic actuator structurecomprising a stator formed by a pair of opposed magnets defining a gaptherebetween a floater formed by a flat actuator coil interposed in thegap. The gap has a width d and the flat coil has a thickness d_(c) andthe ratio of coil thickness d_(c) to gap width d is between ⅓ and ½(d_(c)/d=⅓ to ½) to provide a larger rattle space without sacrificingforce applied between the coil and magnets.

It is a further object of the present invention to provide a newactuation system geometry, providing a well conditioned transformationfrom actuator currents to resultant forces and torques, to produceuniformly distributed commanded forces and torques.

Broadly the present invention relates to a joystick structure comprisinga stator formed by an outer cage with an internal cube shapedcompartment and a cube mounted within the compartment with each cubeface of said cube in opposed relation with a corresponding face of saidcompartment, opposing pairs of magnet assemblies one magnet assembly ofeach pair mounted on a cube face and the other on the correspondingopposed face of said compartment, each said pair of magnet assembliesdefining a gap therebetween, a flotor formed by a plurality of flatactuating coils held in fixed relationship with respect to each otherand positioned one of said coils in each of said gaps, said magneticassemblies on three of said cube faces forming a vertex being orientedwith their longitudinal axes substantially parallel to the cubediagonals emanating from said vertex, and said assemblies on theremaining three of the cube faces forming an opposite vertex beingoriented with their longitudinal axes substantially perpendicular to thecube diagonals emanating from said opposite vertex and each said coilshaving their longitudinal axes substantially parallel to thelongitudinal axis of said pair of magnet assemblies between which it isinterposed.

Preferably said stator compartment and said cube are arranged with oneof their major diagonal axes substantially vertical.

It is yet another objective of the present invention to provide a newoptical position sensor that is significantly less expensive thansensing systems used in other maglev devices, even though it providessimilar sensing volume and resolution.

Broadly the present invention relates to an optical position sensorcomprising at least one linear position-sensing diode mounted on one ofa stator or a floater and a plurality of planar light beam projectors onthe other of the stator and flotor with the light beam projectorsprojecting light beams at an angle to each other, said angle beinggreater than zero and said light beams traversing said at least onelinear light position sensor at a plurality of spaced locations when ina field of operation of said optical position sensor and means forseparately identifying said light beams.

Preferably said means for separately identifying said light beams isselected from the group of means for activating said light beams one ata time and means for varying intensities of said light beams atdifferent frequencies.

Preferably said linear light position sensor are mounted in the sameplane.

Preferably said plurality of planar light beams comprises two and saidtwo light beams are substantially perpendicular.

Preferably said linear light position sensors are arranged in a plane asa triangle and said plurality of light beams comprises three planarlight beams arranged to project light along the adjacent faces of apyramid with its vertex pointing towards said plane and projecting alight triangle onto said plane.

Preferably said linear light position sensor triangle is substantiallyequilateral and said pyramid is substantially a cubic pyramid.

Preferably said light is infrared light.

BRIEF DESCRIPTION OF THE DRAWINGS

Further feature, objects and advantages of the present invention will beevident from the detailed description of the present invention taken inconjunction with the accompanying drawings in which;

FIG. 1 is an isometric illustration of a flat actuating coil (Lorentzvoice coil) positioned between a pair of opposed cooperating magnetsthat forms the basic actuator structure of the present invention.

FIG. 1A is a plan view of the voice coil of FIG. 1 indicating thevarious dimensions used to optimize the design.

FIG. 2 is a plot of Maximum force vs. magnetic gap d showing the effectof gap and coil thickness on the force that may be generated.

FIG. 3 is an exploded view of the structure the actuator device(joystick) of the present invention with parts omitted but showing theoptical sensor of the present invention mounted in sensing position.

FIG. 4 is a view similar to FIG. 3 but with the sensor omitted.

FIG. 5 is a partially exploded plan view of the structure of theactuator device with parts omitted.

FIG. 6 is a schematic illustration of vectors aligned with the coilcurrent directions generated by operation of the coils in the structuralarrangement illustrated in FIGS. 3, 4 and 5.

FIG. 7 is an exploded view of the beam generator portion of the opticalsensor of the present invention.

FIG. 7A is a section along the line A—A of FIG. 7.

FIGS. 8 and 9 are schematic views illustrating the operation of theoptical sensor of the present invention.

FIG. 10 is an illustration of a system for enlarging the field of thesensor by using multiple position sensing diodes (PSD) and staggeringtheir positions.

FIG. 11 illustrates the operation of an optical sensor for measuring twodegrees of freedom using a single PSD and two planar light beams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF PRESENT INVENTION

Basic Actuator Structure

The basic actuator structure 10 as shown in FIGS. 1 and 1A is composedof a flat coil 100 interposed in a magnetic assembly 12 between a pairof cooperating magnet assemblies 110 and 120. Each magnet assembly iscomposed of a pair of magnets 112 and 114 mounted on and coupledtogether via magnetically permeable (soft iron, for example) returnplates 116 to generate a magnetic field as indicated by the arrows B.The field interacts with the coil current as indicated by the arrows Ito generate a Lorentz force as indicated by the vector F. For clarity,the coil 100 is shown translated upwards along the direction of itsforce F, but its nominal position is in the middle of the magneticassembly.

In the system illustrated in FIG. 1 the various dimensions of theelements have been designated as follows; the magnet width w_(m), themagnet thickness t_(m), the magnet length l_(m), the magnet spacingg_(m), the coil 100 thickness d_(c), the magnetic gap length d, and thecoil 100 “rattle space” between the adjacent opposed magnets 112 and 114measured in a direction orthogonal to the coil 100 is designated d_(r)and is equal to d−d_(c) i.e. d_(r)=d−d_(c).

The coil 100 as indicated in FIG. 1A is formed around a core (may besimply an air space) to form a core or gap 118 having a lengthdesignated as l_(c) a gap width g_(c) and a coil width w_(c).

The coil can be wound by using flat or other conducting material wire.For the purposes of defining (calculating the dimensions for) basestructure 10 the coil wire length has been designated l_(wire), and thecoil “packing efficiency” η_(pack), η_(pack)=s_(eff)/s_(wire), the ratioof conducting cross sectional area s_(eff), to total cross sectionalarea, that is conducting plus insulating, cross-sectional area s_(wire).Note that η_(pack) depends on the wire cross-sectional shape (bestpacking achieved by flat wire), and the ratio of insulating material toconducting material.

For the purposes of determining the design formula (equation 6 below)let p be the coil conductor resistivity; R the coil resistance; P_(coil)the power dissipated in the coil, and I the coil current.

The actuator force is given by Lorentz's law. To obtain the designformula (equation 6 below), it is assumed that

(i)˜the flux crossing the coil is a constant B_(g),

(ii)˜fringing fields are negligible, i.e., the flux outside the magnetprojection through the coil is negligible. Implicit in this assumptionis that the magnet projection through the coil is completely containedin the coil, i.e., g_(c)≦g_(m), l_(m)≦l_(c), and w_(m)+g_(m)≦w_(c)+g_(c)

For the actuator in FIG. 1, given assumption (ii), the length of wirethat produces a force is given by l_(eff)=2l_(m)w_(m)d_(c)/s_(wire).

Then, if η_(geom)=2l_(m)w_(m)d_(c)/(s_(wire) l_(wire))=(coil volumeproducing force/total coil volume) is an efficiency factor determined bythe coil geometry, we obtain the following expression for the actuatorforce: $\begin{matrix}{{F\left( {B_{g},d,l_{m},w_{m}} \right)} = {B_{g}{Il}_{eff}}} & (1) \\{= {B_{g}\sqrt{\frac{P_{coil}}{R}}\quad \frac{2l_{m}w_{m}d_{c}}{s_{wire}}}} & (2) \\{= {B_{g}\sqrt{\frac{P_{coil}}{\frac{\rho \quad l_{wire}}{\eta_{pack}s_{wire}}}}\quad \frac{2l_{m}w_{m}d_{c}}{s_{wire}}}} & (3) \\{= {B_{g}\sqrt{\eta_{pack}}\quad \sqrt{\frac{P_{coil}}{\rho}}\quad \sqrt{\frac{1}{s_{wire}l_{wire}}}\quad 2l_{m}w_{m}d_{c}}} & (4) \\{= {B_{g}\sqrt{\eta_{pack}}\quad \sqrt{\eta_{geom}}\quad \sqrt{\frac{Pcoil}{\rho}}\quad \sqrt{2w_{m}l_{m}d_{c}}}} & (5) \\{= {B_{g}\sqrt{\eta_{pack}}\quad \sqrt{\eta_{geom}}\quad \sqrt{\frac{P_{coil}}{\rho}}\quad {\sqrt{2w_{m}{l_{m}\left( {d - d_{r}} \right)}}.}}} & (6)\end{matrix}$

For the coil shown in FIG. 1,

η_(geom)=2l _(m) w _(m)/(2l _(m) w _(m) +πw ² _(m)+2w _(m) g _(c) d_(c))≈2l _(m) w _(m)/(2l _(m) w _(m) +πw ² _(m)),

and is approximately 60% when l_(m)=20 mm, w_(m)=8 mm. Packingefficiencies for conventional coils (round copper wire) are about 75%,while flat copper coils reach efficiency values close to 95%.

Assuming that the actuator flux in FIG. 1 is steered perfectly by thesoft iron backplates, the field in the center of the gap aligned withthe center of the magnet can be calculated by replacing the actuatormagnets with equivalent solenoids and using the Biot-Savart Law. see{Magnet Sales \& MFG Co. 1996 Catalog}, “High performance permanentmagnets.” 11248 Playa Court, Culver City, Calif. 90230 which isincorporated herein by reference. $\begin{matrix}\begin{matrix}{{B_{g}\left( {d,l_{m},w_{m},t_{m}} \right)} = \quad {\frac{2B_{r}}{\pi}\quad\left\lbrack {{\tan^{- 1}\quad \frac{w_{m}l_{m}}{d\sqrt{d^{2} + l_{m}^{2} + w_{m}^{2}}}} -} \right.}} \\{\quad {\tan^{- 1}\frac{w_{m}l_{m}}{\left( {{4t_{m}} + d} \right)\sqrt{\left( {{4t_{m}} + d} \right)^{2} + l_{m}^{2} + w_{m}^{2}}}}}\end{matrix} & (7)\end{matrix}$

where B_(r) is the magnetic material residual flux.

Substituting (7) in (6), one can relate the actuator dimensions to theresulting force. An additional lower bound of the form t_(s)≧αw_(m) canbe imposed on the iron return plates thickness in order to avoidsaturation, where a is a constant determining the maximum relative widthof iron return plate 120 to magnet 110 that avoids saturation. Withappropriate inequality constraints to account for the desiredgeometrical dimensions, e.g.,

2t _(s)+2t _(m) +d≦d _(max),

a maximum actuator thickness, or l_(m)+2w_(m)≦a_(max), a maximum flotorcube size, t_(m), w_(m), l_(m), and d that a maximize actuator force canbe obtained by solving a nonlinear mathematical program as described ina number of available texts. The geometrical efficiency η_(geom)increases with the ratio l_(m)/w_(m), (or l_(c)/w_(c)) whilel_(m)+2w_(m) will be bounded by flotor size.

In practice, l_(m), w_(m) are often selected separately as a function offlotor geometry, desired motion and force range and desired forcelinearity. Then the magnet thickness t_(m) and the magnetic gap dareselected by substituting equation (7) into (6), plotting the actuatorforce

F(B _(g)(d,t _(m)))

as a function of the magnetic gap d for a number of magnet thicknessest_(m), and choosing the maximizing magnetic gap d. The optimal coilwidth is recovered as d_(c)=d−d_(r). This is the best coil-width for agiven coil rattle space. By “best coil” we mean the one that generatesthe highest force for a given power dissipated in the coil, and byrattle space we mean the range of translation of the coil in a directionnormal to its surface (parallel to the gap). The raffle space determinesthe motion range of the flotor.

Such a plot is shown in FIG. 2, obtained from (7) and (6) with magnetdimensions l_(m)=20 mm, w_(m)=8 mm, t_(m)=4 mm, and required rattlespace d_(r)=6 mm. The optimum gap length is found to be d_(optimal)=10.8mm, with corresponding optimum coil width of d_(c) _(optimal) =4.8 mm.This leads to a coil width to magnetic gap width of the order of 1:2.

It is to be noted that the coil width to magnetic gap ratio issubstantially larger in this design than in all other reported maglevjoystick designs (Hollis et. al., 1991), (Salcudean et. al., 1995),(Berkelman et. al., 1996) all of which are referred to above andincorporated herein by reference. Indeed, prior coil width to magneticgap ratios are of the order of 1:7. Coils with such gap ratios arenowhere near as efficient as larger coil width to gap ratios of thepresent invention namely greater than 1:3.

Furthermore, note that the above formulation does not involve the coilresistance, only the power dissipated in it, its resistivity andgeometrical properties. Thus the coil resistance can be selected formaximum power transfer from the power amplifier after finding itsdimensions. The wire gauge can then be selected to give R=8 Ω in orderto match available current drivers. Although the predicted actuatorforce is correct only in the middle of the gap, when the flotor is inits nominal center, the magnetic field formula (7) can be extended togive the field along the magnet center line (see Magnet Sales, 1996) andlead to a more accurate computed actuator force. However, it was foundthat predicted force and field values compare well with experimentalones with small errors (less than 5%) in several actuator designs ofvarious sizes.

The coil design optimization approach presented above has areas ofapplication outside magnetic levitation systems. Indeed, coreless motorssuch as the Maxon 80 motors can be optimized for torque in low-speedapplications in a similar manner, loudspeaker coils, especially woofersand sub-woofer can be optimized in a similar fashion.

Joystick Structure

The joystick device has six basic actuators as used before in a numberof designs (see Hollis et. al., 1991 referred to above and incorporatedherein by reference), however it is preferred to employ the improvedbasic actuator as described above in conjunction with FIGS. 1, 1A and 2.

As shown in FIGS. 3, 4, and 5 in exploded condition the flotor 103 ofthe present invention is formed as a cubic shell formed by couplingtogether an upper structure 104 and a lower structure 108 each formingin effect ½ the cube. The cube halves 104 and 108 are in the illustratedarrangement secured together by bolts (not shown) passing through thebolt holes 106 (holes 106 on the part 108 in the illustrated arrangementare threaded).

Each of the cube halves 104 and 108 are formed by 3 symmetricallypositioned coil mounting portions 104A, 104B, 104C and 108A, 108B and108C respectively each of which has a coil receiving aperture 105 inwhich the coils (not shown) are oriented so that their longitudinal axes107 (see FIGS. 1 and 1A) are along the diagonals of their respectiveeach cube face portions 104A, 104B, 104C, 108A, 108B and 108C. It willbe noted that the coils 100 on the portion 108 (in rest position) willhave their longitudinal axes substantially horizontal and that thelongitudinal axes 107 of those in portion 104 are in planessubstantially perpendicular to the those in the bottom portion 108.

In its nominal or rest position, the cubic flotor structure 103 iscentered within the magnetic gaps with one of its main diagonals(centerline 109) being vertical.

The stator structure 135 includes plates 124 and 126 which mount theouter magnetic assemblies 110 (outside the shell of flotor 103) and aninternal cube 128 which mounts cooperating internal magnet assemblies120 (internal of the shell of the flotor 103) and of course theirmountings.

The cube 128 on which the internal magnet assemblies 120 are mounted issupported with one of its main diagonals vertical. In the rest positionof the flotor 103 this main diagonal substantially aligned with the mainaxis 109 of the flotor structure and therefore is designated by the samereference number 109. The cube 128 is supported in this position bythree cylindrical posts 130 attached to support structures 132. Theposts 130 pass through passages through the floater 103 formed bytruncating the portions 108A, 108B and 108C as indicated at 111 (seeFIGS. 3 and 4).

The internal magnet assemblies 120 are mounted along the diagonals of apermeable iron cube 128. Magnet assemblies 120 on the lower half of thecube 128 (cooperating with the portion 108) are oriented with theirlongitudinal axes (axis extending substantially perpendicular to thedimension g_(m) designating the width of their gaps) extendingsubstantially horizontal i.e. perpendicular to the axis 109 and thelongitudinal axes of the magnet assemblies on the upper half(cooperating with portion 104) extending along substantially verticalplanes i.e. intersect to the axis 109. In other words the magneticassemblies mounted on one set of three cube faces of the cube forming avertex are oriented with their longitudinal axes substantially parallelto the cube diagonals emanating from that vertex, and the magneticassemblies on the remaining three of the cube faces that form theopposite vertex are oriented with their longitudinal axes substantiallyperpendicular to the cube diagonals emanating from the opposite vertex.

Magnet assemblies 110 external to the coil shell 103 are mounted oncantilevered permeable iron plates 124 and 126 which combine to form acubic chamber or compartment in which the cube 128 and its magnetassemblies 120 are contained so that each face of the cube 128 isopposite its corresponding wall of the cubic compartment. Each externalmagnet assembly 110 is mounted on the plate 124 or 126 in opposingposition with the internal magnet assembly 120 on the opposing face ofthe cube 128 (i.e. the face of the cube 128 facing its plate 124 or 126)and thus the cooperating magnet assemblies 110 and 120 on each pair ofopposing faces of compartment formed by the plates 124 and 126 theopposing face of the core 128 are positioned to cooperate and act on thecoil 100 (not shown in FIGS. 3, 4 or 5) interposed therebetween i.e. thecoils 100 on upper shell portion 104 cooperate with magnet assemblies110 and 120 on the plate 124 and upper half of the cube 128 and thecoils 100 on the lower portion 108 cooperate with the magnet assemblies110 and 128 on the permeable iron plates 126 and the lower faces of thecube 110.

Each coil 100 is oriented with respect to the magnet assemblies 110 and120 between which it is interposed with its longitudinal axis 107substantially parallel to the longitudinal axis of its assemblies 110and 120.

The magnetic field return plates 124 mount on the magnetic field returnplates 126 which in turn are attached to a ring 134 and posts 136mounted onto the base of the device. The posts 132 are also mounted onthe ring structure 134 to form the stator structure 135.

A printed circuit board(PCB) 180 (see FIG. 3) fits under the base 134 ofthe stator 135 formed by the plates 124 and 126 and posts 132 mountingthe cube 128 and the base 134. The circuit board 180 carries the devicesensing and power electronics and a microcontroller as will be describedbelow.

For clarity, FIG. 3 shows the iron cube 128 internal to the cube withthe magnets 120 attached to it, while FIG. 4 shows the magnets 110attached to the external cube but not the external magnets 120.

As shown in FIGS. 3, 4 and 5, because the flotor coils are arrangedalong the diagonals of a cube as shown, four of the cube vertices can becut away i.e. the cut aways 111 described above and a large cut in thebottom vertex of the lower flotor structure 108 as indicated at 113 inFIG. 4.

A handle or other payload (not shown) can be mounted on a vertical shaftpenetrating the top vertex of the upper flotor structure 104. Thesupport posts 130 shown in FIGS.˜3 and 5 may be set to locate the statorrelative to the PCB and sensor, closer to one of the PCB sides, thusallowing a tapered surface for the user's forearm and wrist to rest on.

The cut vertices are also shown schematically in FIG. 6.

The magnetic gaps allow the flotor 103 to translate and rotate inarbitrary directions and about arbitrary axes, respectively, within theconfines of the magnetic gaps.

The rotational and translational workspaces do not decouple in thisdesign, unlike in the spherical geometry suggested in (Berkelman et.al., 1996 referred to above). However, the angular motion of the flotoris substantial (typically ±10°) from nominal, and is actually limited ina current model by the optical sensor range, not by mechanicalinterference.

An optical position sensor a preferred form of which will be describedhereinbelow fits under the flotor structure 108 and projects lines ontoposition sensing diodes mounted on the circuit board 180.

Device Modeling

The device modeling used for the present invention follows closely theapproach from Hollis et. al., 1991 referred to above and incorporatedherein by reference.

The flotor is represented schematically in FIG. 6 . . . C₁ through C₆denote the actuator coil centers. I₁ through I₆ are parallel to thecurrents flowing through the straight parts l_(c) of the coils 100.

In the nominal position, the resultant force-torque vector acting on theflotor is computed by summing the forces f_(i) and torques FC_(i)×f_(i),where f_(i) is computed by taking the vector product I_(i)×B_(i) of thecurrent vectors I₁, . . . , I₆ and the magnetic field vectors B₁, . . ., B₆ (these are outward normals to the faces of the cube and are notshown). These calculations are first done with respect to a vertexcoordinate system centered at V_(C) with unit vectors along the cubeedges, then they are transformed to the coordinate system {F,[i_(F),j_(F), k_(F)]} located at the center of the cube and having k_(F)aligned with the vertical (in FIGS. 3 and 4 the plane of vertices V_(A),V_(B), V_(C) is horizontal). A resultant 6×6 matrix A mapping unit coilcurrents to flotor forces and torques can be obtained in this coordinatesystem from the device geometry and an actuator force gain. For example,if the cube has its side equal to 50.5 mm and the predicted actuatorforce per unit of current is of 4 N/A, the transformation matrix A isgiven by: $A = \begin{bmatrix}{- 1.15} & 2.31 & {- 1.15} & {- 3.46} & 3.46 & 0 \\{- 2.00} & 0 & 2.00 & {- 2.00} & {- 2.00} & 4.00 \\{- 3.27} & {- 3.27} & {- 3.27} & 0 & 0 & 0 \\{- 0.10} & 0 & 0.10 & 0.03 & 0.03 & {- 0.07} \\0.06 & {- 0.12} & 0.06 & {- 0.06} & 0.06 & 0 \\0 & 0 & 0 & 0.09 & 0.09 & 0.09\end{bmatrix}$

The first three rows of A have units of N/A, the next three Nm/A. It isworthwhile noting that A has two groups of equal singular values 5.66,5.66, 5.66 N/A and 0.16, 0.16, 0.16 Nm/A corresponding to forces andtorques, their scaled values giving the maximum forces and torques thatwould be obtained with a given power supply if all coil resistances wereidentical. Thus this geometry distributes the power load across theactuators in a uniform manner.

Optical Position Sensor

A novel optical sensor 139 to determine the position and orientationoffsets of the flotor 103 with respect to the stator 135 will now bedescribed with reference to FIGS. 3 and 7 through 11.

As shown in FIG. 3, the optical position sensor consists of a planarinfrared beam generator 140 mounted on the flotor 103, and three linearlight position sensors, such as, linear lateral effect position sensingdiodes (PSD's)—PSD A, PSD B and C, mounted on the PCB plane 180 underthe flotor 103.

An embodiment of an infrared beam generator is explained in detail inFIGS. 7 and 7A. A number (two shown) of wide angle light emitting diodes(LED's) 142 are mounted on each of the three mounting faces 141 of anLED holder 144 with 120 degree symmetry. Deep and wide slits 143perpendicular to the mounting faces are cut in the LED holder allowinginfrared light to emanate along orthogonal planes 160 (illustrated inFIG. 8) of a light cube.

Two precision masks 146 and 150 with slits 156 are used to insure thatthe light planes emanating through the wide slit 143 have littledivergence and are precisely aligned to form the faces of a cubicpyramid. Precise alignment can be achieved by the use of locator pins155 as illustrated in FIG. 7.

As shown in FIGS. 8 and 9, the light planes 162 coincide with the facesV_(A) V V_(B), V_(B) V V_(C), and V_(C) V V_(A), of a cube. Note that inthis particular device design, these faces are parallel to the faces ofthe lower flotor structure 108, but that is not necessary for otherdesigns.

Other ways of generating orthogonal light planes along the faces of acube are possible and are obvious to those skilled in the art.

The PSDs PSD A, PSD B and PSD C are mounted on the PCB plane 180 alongthe sides of an equilateral triangle.

The operation of PSDs is well known—see, for example, (U.S. Pat. No.4,785,180 Dietrich et. al., or {Dietrich J., Plank, G.},“{Optoelectronic System Housed in a Plastic Sphere},” 1988. The positionof a focused light beam (preferably infrared) projected onto the PSDactive area can be obtained by measuring the PSD electrode currents.Position along the PSD axis is computed by dividing the differencebetween the electrode currents by the sum of the electrode currents.

The optical position sensor is controlled by the board 180 and operatesas follows (see FIGS. 3, 8 and 9). Each of the light planes 160 areturned on in sequence, first V_(A)VV_(B), then V_(B)VV_(C), thenV_(C)VV_(A), at high frequency, by a microcontroller. When V_(A)VV_(B)is ON, the positions of its intersections P₁ and P₂ with PSD A and PSDB, respectively, are detected by measuring the PSD currents insynchronization with the flashed light plane. When V_(B)VV_(C) is ON,the positions of its intersections P₃ and P₄ with the active areas ofPSD B and PSD C, respectively, are detected. When V_(C)VV_(A) is ON, thepositions of its intersections P₅ and P₆ with PSD C and PSD A,respectively, are detected.

In a particular embodiment of this optical position sensor, the nominalflotor position with respect to the PSDs can be defined such that pointsP₁ and P₆ collapse into one point, and so do the pairs P₂, P₃ ad P₄, P₅as shown in FIG. 9.

The position of the moving flotor with respect to a coordinate systemattached to the stator (and the PCB) can be obtained from coordinates ofthe points P_(i), i=1, . . . , 6 in the PCB plane 180. First, theintersections A of line P₁P₂ with P₅P₆, B of P₁P₂ with P₃P₄, and C ofP₃P₄ with P₅P₆, are computed.

As shown in U.S. Pat. No. 5,059,789 issued to Salcudean, October 1991,the light vertex V can be computed easily as the point of intersectionof three spheres of diameters ∥AB∥, ∥BC∥, ∥CA∥, passing through A and B,B and C, and C and A, respectively. Indeed, the loci of V such that theangles (VA, VB), (VB, VC) and (VC, VA) are 90° are spheres withdiagonals AB, BC and CA. Once V is located, [i_(v), j_(v), k_(v)] can becomputed by normalizing VA, VB, VC.

Note that similar sensors can be constructed by using three light planesthat intersect three linear PSDs (or linear CCDs) and solving theassociated direct kinematic equations numerically via an iterativemethod such as Newton's method. In general, a system of six equationswith six unknowns describing a geometry consistent with the measurementshas to be solved. If the light planes are along the faces of a pyramid,the intersection of three thori needs to be computed.

The optical sensor described above has a number of advantages. Unlikeprior LED-PSD-based sensors used in previous maglev wrists (Hollis et.al., 1991),(Berkelman et. al., 1996), and disclosed elsewhere(Salcudean, 1991) all referred to above, linear, not two-dimensional,PSDs are necessary. Also since the PSDs may be in the same plane, theycan be mounted on a single printed circuit board, with better alignmentand manufacturability than would be the case if a three-dimensional PSDstructure were used.

The sensor embodiment described above relies upon time-domainmultiplexing to sense the position of each of the light planes on eachof the PSDs. It will be apparent that the same can be accomplished byfrequency-domain multiplexing, where the light planes vary in intensityat different frequencies. Multiple light plane projections on the PSDscan be recovered by known techniques of filtering and signal processing.

The sensor embodiment described above relies on using just three PSDs.In order to enlarge the volume of detection, multiple PSDs can bestaggered as shown in FIG. 10.

The multiplexing concept described above can be used to measure motionin fewer degrees of freedom. For example, a two-dimensional planartranslation sensor can be realized by using one PSD, and two multiplexedplanar beams projected on its surface, as shown in FIG. 11. When lightplane AA′ is ON, the PSD detects motion essentially orthogonal to AA′.When light plane BB′ is ON, the PSD detects motion essentiallyorthogonal to BB′. The position of the carrier of AA′ and BB′ is easilydetermined in this way.

Note that the above description of the optical position sensor andvariants obvious to those skilled in the art may be used in many otherinstances, not only for position and orientation sensing for activejoysticks. Motion can be detected for vibration isolation, asix-degree-of-freedom head tracker can be developed by using an infraredplane generator attached to the head of an operator and linear detectorson the ceiling, etc.

Controller Board

The novel actuator geometry, novel optical position sensor and novelactuator optimization presented above allow a designer to fit all theelectronics components of a magnetically levitated device on a smallcontroller board. For example, a 10.6″×5″ PCB comprising the following:

analog electronics to amplify the PSD currents

LED transistor drivers

analog-to-digital converters to read the input from the PSDs, withenough spare channels to accommodate the outputs of asix-degree-of-freedom force sensor that could be mounted on the flotor

Pulse-Width-Modulation (PWM)-driven H-bridges for the coils

one small fan for occasional forced air cooling,

two serial and one parallel communication ports—a 50 MHz Intel 80C196NUmicrocontroller with associated EPROM and RAM.

The serial port can be used as a fast synchronous link allowingreal-time control by a remote host. A second serial port is provided forthe use of debugging tools. Several host communication methods are beingprovided, even though they take a significant amount of PCB space, forflexibility and exploration of the best approach to host connection.Other host communications standards such as the Universal Serial Bus andthe MIDI interface can be used.

The microcontroller performs basic I/O communications with a host. Itgenerates the time-multiplexed light planes needed for optical sensingand the PWM signals needed to drive the coils, and it computes the basiccontrol functions (direct kinematics, wrench vector computationaccording to a control law for mechanism emulation, and transformationof the control wrench into equivalent coil forces or currents). A flotorcontroller diagram is taught in (U.S. Pat. No. 4,874,998 issued toHollis in October 1989) and in Hollis et. al., 1992 referred to above.

Haptic Interface Application

The range 6-DOF magnetically levitated device disclosed above, includingnovel geometry and packaging, optical sensor, and actuator optimizationis envisaged as being used for example as

(i) an intelligent haptic interface emulating simple mechanisms from itsown library or downloaded from the host computer, such as limit stops,gimbals, sliders, various friction forces and simple geometricconstraints that can be computed using its fixed-point microcontroller,

(ii) a “dumb” haptic interface or teleoperation master, with themicrocontroller board acting as an input-output board and mostcalculations being performed by the host or another external computer.The former mode does not require high communication rates with the hostcomputer, while the second one does, as low-level data such asstiffnesses and forces are being passed between the host and themicrocontroller board.

The characteristics of a specific device constructed in accordance withthe present invention are summarized in Table 1:

TABLE 1 Summary of desk-top maglev joystick characteristics. Moving mass260 grams Motion range ±3 mm, ±5° Maximum acceleration >10 g Maximumcontinuous force 16N Peak force 34N Power consumption for levitation 1.6W Optical sensor resolution approx. 10 microns Isotropic design Opticalsensor one order of magnitude cheaper than used in previous designsOptimized actuator over 50% more efficient than in previous designs Allelectronics except power supply integrated in the base with a 10.5″ ×5.5″ footprint

The haptic interface has a motion range that exceeds that of many if notall similar passive devices, has high acceleration and force capability,and fits, complete with all electronics and a microcontroller, into asmall enclosure tapering down from a handle 5.5″ high to a base roughlytwo thirds the size of a sheet of paper.

Although the principle of Lorentz magnetic levitation (Hollis, 1989)teaches an actively levitated structure, it is often advantageous toinclude passive flotor supports.

With significant damping, the range of achievable stiffnesses by activecontrols can be improved, while a passive stiffness restoring the flotorto a nominal center can improve error recovery and safety.

Furthermore, it is recognized that in many applications, not all sixdegrees of freedom are required. Cubic structures and supports forvoice-coil flotors with less than six degrees of freedom can beenvisaged.

Having described the invention modifications will be evident to thoseskilled in the art without departing from the spirit of the invention asdefined in the appended claims.

We claim:
 1. A joystick structure comprising a stator formed by an outercage with an internal cube shaped compartment and a cube mounted withinsaid compartment with each cube face of said cube in opposed relationwith corresponding face of said compartment, opposing pairs of magnetassemblies one magnet assembly of each opposed pair mounted on a cubeface of said cube and the other on a corresponding opposed face of saidcompartment, each said pair of magnet assemblies defining a gaptherebetween, a flotor formed by a plurality of flat actuating coilspositioned in fixed spatial relationship with respect to each other withone of said coils in each of said gaps, said magnetic assemblies onthree of said cube faces forming a vertex being oriented with theirlongitudinal axes substantially parallel to the cube face diagonalsemanating from said vertex, and said assemblies on the remaining threeof the cube faces forming an opposite vertex being oriented with theirlongitudinal axes substantially perpendicular to the cube face diagonalsemanating from said opposite vertex and each of said coils having itslongitudinal axis substantially parallel to the longitudinal axis ofsaid pair of magnet assemblies between which it is interposed, said cubehaving a major cube diagonal extending between said vertex and saidopposite vertex.
 2. A joystick structure as defined in claim 1 whereinsaid stator compartment and said cube are arranged with said major cubediagonal axis substantially vertical.
 3. A joystick structure as definedin claim 1 wherein each said gap has a width d and each said flatactuator coil has a thickness d_(c) and wherein the ratio of coilthickness d_(c) to gap width d is at least ⅓.
 4. A joystick structure asdefined in claim 3 wherein said ratio of coil thickness d_(c) to gapwidth d is between ⅓ and ½.
 5. A joystick structure as defined in claim2 wherein each said gap has a width d and each said flat actuator coilhas a thickness d_(c) and wherein the ratio of coil thickness d_(c) togap width d is at least ⅓.
 6. A joystick structure as defined in claim 5wherein said ratio of coil thickness d_(c) to gap width d is between ⅓and ½.
 7. A joystick structure as defined in claim 1 further comprisingan optical position sensor comprising at least one linear light positionsensor mounted on one of said stator and said flotor and a plurality ofplanar light beam projectors mounted on the other of said stator andsaid flotor, said planar light beam projectors being arranged to directbeams of light at an angle to each other, said angle being greater thanzero and said beams of light traversing said at least one linear lightposition sensor at a plurality of spaced locations when in a field ofoperation of said optical position sensor.
 8. A joystick structure asdefined in claim 2 further comprising an optical position sensorcomprising at least one linear light position sensor mounted on one ofsaid stator and said flotor and a plurality of planar light beamprojectors mounted on the other of said stator and said flotor, saidplanar light beam projectors being arranged to direct beams of light atan angle to each other, said angle being greater than zero and saidbeams of light traversing said at least one linear light position sensorat a plurality of spaced locations when in a field of operation of saidoptical position sensor.
 9. A joystick structure as defined in claim 4further comprising an optical position sensor comprising at least onelinear light position sensor mounted on one of said stator and saidflotor and a plurality of planar light beam projectors mounted on theother of said stator and said flotor said light beam projectorsprojecting beams of light at an angle to each other, said angle beinggreater than zero and said beams of light traversing said at least onelinear light position sensor at a plurality of spaced locations when ina field of operation of said optical position sensor.
 10. A joystickstructure as defined in claim 7 wherein said at least one linear lightposition sensor comprises a plurality of said linear light positionsensors all of which are mounted in the same plane.
 11. A joystickstructure as defined in claim 10 wherein said plurality of light beamprojectors comprises two of said light beam projectors mounted so thatsaid beams of light projected by said two light beam projectors aresubstantially perpendicular.
 12. A joystick structure as defined inclaim 10 wherein said plurality of linear light position sensorscomprises three linear light position sensors and said linear lightposition sensors are arranged in said plane as sensing triangle and saidplurality of light beam projectors comprises three planar light beamprojectors arranged to project beams of light along the adjacent facesof a pyramid with its vertex pointing towards said plane and projectinga triangle of light on said plane.
 13. A joystick structure as definedin claim 12 wherein said sensing triangle is an equilateral triangle.14. An optical position sensor comprising three linear light positionsensors arranged in a plane with their axes forming a light sensingtriangle mounted on one of a flotor and a stator and three planar lightbeam projectors mounted on the other of the stator and flotor with saidplanar light beam projectors projecting light beams, said light beamsbeing arranged at an angle to each other, said angle being greater thanzero and said light beams traversing said three linear light positionsensors at a plurality of spaced locations when in a field of operationof said optical position sensor.
 15. An optical position sensor asdefined in claim 14 wherein said three planar light beam protectors arearranged to project planar light beams along the adjacent faces of apyramid with its vertex pointing towards said plane and projecting atriangle of light on said plane.
 16. An optical position sensor asdefined in claim 15 wherein said light sensing triangle is anequilateral triangle.
 17. An optical position sensor as defined in claim14 wherein said light sensing triangle is an equilateral triangle.